Advanced Toroidal Facility Torsatron Research Articles

Microwave scattering diagnostic on the Advanced Toroidal Facility torsatrona)

A 2 mm scattering diagnostic used to study density fluctuations of the electron cyclotron resonance heated plasma in the Advanced Toroidal Facility is described. A four‐channel flexible antenna design was chosen to monitor fluctuations within the wave number range of 3<k⊥<20 cm−1 and to locate the scattering volume anywhere in the stellarator minor cross section. Special attention was paid to the system calibration and to the normalization of the k spectra. Analysis of the diagnostic noise showed that the main source of spurious signal was a scattering from high‐level edge turbulence driven by the second‐harmonic radiation from the gyrotron used for plasma heating. This spurious signal was suppressed using waveguide filters and rearrangement of the microwave circuit elements. Evidence in support of direct scattering from a well‐defined plasma volume was found. Good correlation of the scattered waves’ intensity with the density scale length was observed. Use of a dual‐homodyne circuit configuration made is possible to determine that the plasma density fluctuations were propagating in the electron diamagnetic direction.

Experimental evidence of three-wave coupling on plasma turbulence.

Experimental evidence of the nonlinear nature of the broadband edge fluctuations has been obtained in edge turbulence in the Advanced Toroidal Facility torsatron. Whereas little nonlinear wave interaction is found in the scrape-off layer region, three-wave coupling is enhanced in the plasma edge region (r<α shear ). The degree of three-wave coupling strongly depends on the plasma conditions; it decreases in the temperature range (T e ≃10 eV) where the ionization rates depend strongly on T e suggesting a link between ionization source and turbulence

Effects of limiter biasing on the ATF torsatron

Positive limiter biasing on the currentless Advanced Toroidal Facility (ATF) torsatron produces a significant increase in the particle confinement with no improvement in the energy confinement. Experiments have been carried out in 1-T plasmas with {approximately}400 kill of electron cyclotron heating ECM. Two rail limiters located at the last closed flux surface (LCFS), one at the top and one at the bottom of the device, are biased at positive and negative potentials with respect to the vessel. When the limiters are positively biased at up to 300 V, the density increases sharply to the ECH cutoff value. At the same time, the H{sub {alpha}} radiation drops, indicating that the particle confinement improves. When the density is kept constant, the H{sub {alpha}} radiation is further reduced and there is almost no change in the plasma stored energy. Under these conditions, the density profile becomes peaked and the electric field becomes outward-pointing outside the LCFS and more negative inside the LCFS. In contrast, negative biasing yields some reduction of the density and stored energy at constant gas feed, and the plasma potential profile remains the same. Biasing has almost no effect on the intrinsic impurity levels in the plasma.

Recent results from the ATF torsatron

Recent experiments in the Advanced Toroidal Facility (ATF) torsatron [Plasma Physics and Controlled Nuclear Fusion Research 1990 (IAEA, Vienna, in press)] have emphasized the role of magnetic configuration control in transport studies. Long-pulse plasma operation up to 20 sec has been achieved with electron cyclotron heating (ECH). With neutral beam injection (NBI) power of ≥1 MW, global energy confinement times of 30 msec have been obtained with line-average densities up to 1.3×1020 m−3. The energy confinement and the operational space in ATF are roughly the same as those in tokamaks of similar size and field. The empirical scaling observed is similar to gyro-reduced Bohm scaling with favorable dependences on density and field offsetting an unfavorable power dependence. The toroidal current measured during ECH is identified as the bootstrap current. The observed currents agree well with predictions of neoclassical theory in magnitude and in parametric dependence. Variations of the magnetic configuration in discharges heated by ECH alone and by NBI change plasma transport and plasma profiles. Magnetic fluctuations respond to the concomitant pressure profile variations. Comparative studies of edge fluctuations in the Texas experimental tokamak (TEXT) [Plasma Physics and Controlled Nuclear Fusion Research 1990 (IAEA, Vienna, in press)] and the ATF stellarator showed remarkable similarity in the levels of fluctuations and the existence of a velocity shear layer.

Plasma fluctuations near the shear layer in the ATF torsatron

Electrostatic turbulence has been investigated in the edge region of the Advanced Toroidal Facility (ATF). A reversal in the poloidal phase velocity of the fluctuations has been observed (velocity shear) which determines a characteristic plasma radius. The location of this shear layer depends on the magnetic configuration, the limiter radius and the plasma conditions. Using the shear position as a reference point, the density fluctuation levels in ATF (currentless stellarator) are very similar to those previously reported in TEXT (ohmically heated tokamak), suggesting that the plasma current is not an important drive for the edge turbulence. The drives for the turbulence appear to be different inside and outside the shear location (ashear), with T̃e/Te < ñ/n in the scrape-off layer (r/ashear > 1) and possibly larger T̃e/Te in the plasma edge edge (r/ashear < 1). There is a spatial decorrelation in the fluctuations at the shear location; this suggests that the poloidal shear flow has an important influence on the edge turbulence. The poloidal correlation length depends on local plasma parameters (e.g. velocity and temperature). When neutral beam injection is added, the high frequency components of ̃n increase.

Magnetic fluctuations in currentless plasmas in the ATF torsatron

Measurements of poloidal magnetic field fluctuations outside currentless, finite-beta (⟨β⟩ ≤ 0.5%, β0 ≤ 3%) plasmas with peaked pressure profiles in the Advanced Toroidal Facility (ATF) torsatron reveal bands of small (B̃θ/Bϕ ∼ 10−5), coherent fluctuations in the frequency range 5-60 kHz. The geometrical structure of these fluctuations shows n = 1 toroidal mode symmetry, with inferred poloidal mode numbers of m = 2 and 3. The coherent Bθ amplitudes increase with ⟨B⟩ for ⟨B⟩ ≲ 0.3%, and then saturate and begin to decrease with ⟨B⟩ for ⟨B⟩ ≳ 0.3%.

Bootstrap-current experiments in a toroidal plasma-confinement device.

The toroidal current observed during electron cyclotron heating in the Advanced Toroidal Facility torsatron is identified as bootstrap current. The observed currents, ranging between +4 and -2 kA, agree well with predictions of neoclassical theory in magnitude and parametric dependence, as determined by systematic scans of quadrupole (shaping) and dipole (magnetic axis shift) moments of the poloidal magnetic field. It has been shown that the bootstrap current in a stellarator can be externally controlled, and zero-current operation can be achieved.

Overview of results from the ATF torsatron

Experiments involving plasma improvement, confinement scaling, bootstrap currents, and edge fluctuations have been carried out in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)]. Average densities n̄e≤9×1019 m−3 have been obtained, with global energy confinement times τ*E≤20 msec. Confinement times generally follow the stellarator/torsatron empirical scaling law, τSL =0.17×P−0.58n0.69eB0.84a2R0.75 (with τSL in seconds, power P in megawatts, density ne in 1020 m−3, and plasma radius a and major radius R in meters). Gas injection during neutral beam injection (NBI) causes increases in ne, so that τ*E does not decrease during NBI. Edge plasma fluctuations are found to exhibit a mode change near the peak of the energy confinement time. Plasma currents observed during electron cyclotron heating have been identified as bootstrap currents.

Second stability in the ATF torsatron—Experiment and theory

Access to the magnetohydrodynamic (MHD) second stability regime has been achieved in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)]. Operation with a field error that reduced the plasma radius and edge rotational transform resulted in peaked pressure profiles and increased Shafranov shift that lowered the theoretical transition to ideal MHD second stability to β0≊1.3%; the experimental β values (β0≤3%) are well above this transition. The measured magnetic fluctuations decrease with increasing β, and the pressure profile broadens, consistent with the theoretical expectations for self-stabilization of resistive interchange modes. Initial results from experiments with the field error removed show that the pressure profile is now broader. These later discharges are characterized by a transition to improved (×2–3) confinement and a marked change in the edge density fluctuation spectrum, but the causal relationship of these changes is not yet clear.

Electron beam and magnetic field mapping techniques used to determine field errors in the ATF torsatron

The beam from an electron gun was used to trace flux surfaces in the Advanced Toroidal Facility (ATF) torsatron. The ATF magnetic field was run steady state at 0.1 T, and the electron beam was detected optically with an image-intensified, solid-state camera when it impinged on a phosphor-coated screen. Closed flux surfaces and islands at several low-order resonances were observed. The largest island, located at the ι= 1/2 surface, was from 5 to 6 cm in width, and its presence implied the existence of magnetic field errors. To determine if these error fields could be traced to small misalignments of the magnetic coils, a device capable of accurately measuring the radial and vertical magnetic field components of individual coil sets was placed in the center of ATF. This device allowed for a determination of the precise location of each of the coils that make up the ATF coil set. No significant coil misalignments were found. A further investigation of the coil configuration led to the identification of dipole fields in the helical field coil leads as the source of the field errors. The techniques developed in making these measurements are described in the text.